I don't think that it makes the car smoke anymore. But having worked on a farm for a while I know that people DO get checked. Its usually farmers that get checked at livestock markets etc - or just anywhere they tend to be in any numbers with road going cars. I guess its because they are considered having most access to it. I also think that anyone who does any work on the car, inspetcs it for insurance purposes etc who notices you've used red diesel has an obligation to report it (not a problem if you are a mechanic - but drive carefully!)
How about using vegetable oil instead? I guess you saw Top Gear.